pgsql: BRIN autosummarization may need a snapshot

BRIN autosummarization may need a snapshot

It's possible to define BRIN indexes on functions that require a
snapshot to run, but the autosummarization feature introduced by commit
7526e10224f0 fails to provide one. This causes autovacuum to leave a
BRIN placeholder tuple behind after a failed work-item execution, making
such indexes less efficient. Repair by obtaining a snapshot prior to
running the task, and add a test to verify this behavior.

Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reported-by: Giovanni Fabris <giovanni(dot)fabris(at)icon(dot)it>
Reported-by: Arthur Nascimento <tureba(at)gmail(dot)com>
Backpatch-through: 13
Discussion: https://postgr.es/m/202511031106.h4fwyuyui6fz@alvherre.pgsql

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f0ad417161977d034e3d5845d043c44bef3d1f8a

Modified Files
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src/backend/postmaster/autovacuum.c | 2 ++
src/test/modules/brin/t/01_workitems.pl | 39 +++++++++++++++++++++++++++++----
2 files changed, 37 insertions(+), 4 deletions(-)